Literature methods for preparing 5,6-dialkyl and 5,6-alkyl-arylpyridine-2,3-dicarboxylates are limited, often requiring oxidation of alkyl or aryl substituents at positions 2 and 3 in order to obtain diacids. R. Jones, J. Am. Chem. Soc. 73, 4380 (1951) describes a method in which reaction of a primary enamine yields 6-alkylpyridine2,3-dicarboxylates which contain electron withdrawing substituents such as COCH.sub.3, CN or CO.sub.2 Et in the 5-position. This method cannot be used to prepare 5,6-dialkyl or alkylaryl pyridine 2,3-dicarboxylates because primary enamines without electron withdrawing substituents cannot readily be prepared, i.e., the reaction of ammonia with aliphatic ketones produces imines which do not tautomerize to enamines and, unless trapped in situ, polymerize. Other methods employing malononitriles such as those described in Japanese Pat. No. 78 69,835 and that of J. I. DeGraw J. Het. Chem. 19, 1461 (1982), can yield 5-alkyl and 5,6-dialkylpyridines but not with the desired 2,3-dicarboxylate substitution directly.
Pyridine-2,3-dicarboxylates are useful intermediates for the preparation of herbicidal 2-(2-imidazolin-2-yl) nicotinic acids, esters and salts such as those disclosed in European patent application No. 81103638.3 filed Dec. 1, 1981, as illustrated in Flow Diagram I below. ##STR4## wherein R.sub.1 is C.sub.1 -C.sub.4 alkyl; R.sub.2 is C.sub.1 -C.sub.4 alkyl or C.sub.3 -C.sub.6 cycloalkyl; and when R.sub.1 and R.sub.2 are taken together with the carbon to which they are attached they may represent C.sub.3 -C.sub.6 cycloalkyl optionally substituted with methyl; W is O or S; and R.sub.3 and R.sub.4 are hydrogen, halogen C.sub.1 -C.sub.6 straight or branched alkyl, alkenyl, or phenyl optionally substituted; R.sub.5 and R.sub.6 are each C.sub.1 -C.sub.4 alkyl.
The condensation with .alpha.,.beta.-unsaturated systems by Michael addition of some halomethylene carbon nucleophiles activated by electron withdrawing groups is well documented, but the corresponding reaction of .alpha.-halo-.beta.-ketoesters such as that of diethyl 3-chloro-2-oxo-butanedioate with acrolein has been reported by P. Bouvier and H. Gault, Bull. Coc. Chim. Fr. 711 (1963), to result in mixtures.
It is an object of this invention to provide a method for the preparation of substituted and disubstituted pyridine-2,3-dicarboxylates and 2-alkyl nicotinates utilizing .alpha.-halo-.beta.-ketoesters and .alpha.,.beta.-unsaturated aldehydes or ketones in the presence of an ammonium salt.